A printhead of an inkjet printing apparatus comprises a plurality of nozzles from which drops of ink are fired on a print media. Sometimes it is necessary to prime one or more printheads by varying a pressure differential to force ink to flow from an ink chamber within the printhead into the nozzles, in order to solve or alleviate problems that may be caused for example by the presence in the nozzles or in its associated firing chamber of dry or crusting ink, air bubbles or foreign particles, or by pigment ink settling.
The pressure differential needed for priming may be varied either by providing a suction effect though a cap sealingly applied around the nozzles on the outer part of the printhead or by increasing the pressure of the ink inside the printhead.
In printers provided with a relatively large ink container arranged remote from the printhead and a relatively small ink chamber within the printhead, in which the ink chamber is continuously fed from the remote container through a supply line, it is known e.g. from U.S. Pat. No. 4,558,326 to provide a positive pressure to the ink in the container in order to purge bubbles and ink contained in the printhead by ejecting ink through the nozzles.
Another known priming method, disclosed in commonly owned U.S. Pat. No. 6,419,343, involves applying a positive pressure to the ink in the ink chamber of the printhead by means of an air pump: the pump is operated by the travel of the printhead carriage to deliver a predetermined volume of gas to the ink chamber.
The predetermined piston stroke and cylinder volume of the pump operated by the travel of the printhead carriage limit the flexibility of the priming operation in terms of pressure and number of printheads primed. Furthermore, each different printing apparatus may require different pressures and thus a different geometry of the pump.
Since the pump is operated by the travel of the carriage, the system also requires a relevant carriage stroke to obtain enough pressure, and this makes it difficult to position the printhead to be primed in relation to the spittoon.